54 ROBERT STANLEY McEWEN 
There were two chief methods used for measuring the re- 
sponse. In the first of these, one fly was tested at a time. It 
was placed in the end of a tube away from the light, and allowed 
to remain for one minute. During that minute a record was 
taken of the furthest distance which it crawled toward the light, 
expressed in inches. If it had not reached the light end, it was 
now put there by the method described above, and the test re- 
peated. This time, however, the furthest inch which it crawled 
away from the light was recorded. Each of these tests was re- 
peated three times, the ends of the tube in which the insects 
started being alternated. An average for the inches crawled 
toward the light in each trial was then taken and an average of 
the inches crawled away from it.! For convenience, the for- 
mer will be referred to as the t. 1. (toward light) average and the 
latter as the f. 1. (from light) average. The algebraic sum of 
these two averages was taken as the t. |. or f. 1. index of the fly 
in question. In any given experiment as many flies as possible 
were thus tested and the sums of the t. |. indices algebraically 
added to the sums of the f. 1. indices. Thus, let us suppose that 
there were four flies in a group. The index of fly 1 was t. 1. 20, 
that of fly 25 %t- 1 30, that’ of fly. 8; tf). 10) and ‘that“of thy 4 
1.5. The index for the group would be expressed as t. 1. 35. 
This method will be designated as method I. 
After using this method for some time it became evident that 
a plan must be devised for testing more flies at a time. This 
seemed desirable because of the extreme variability of individuals, 
despite all efforts to keep environmental factors constant. The 
following plan was worked out for obtaining the phototropic in- 
dex of insects by groups. This system proved far more satis- 
factory than the one described above and has been used in all 
the later and more critical work. It will be designated as 
method II. 
The tube in this case was divided into fifths, and to each fifth 
was assigned an arbitrary value as follows. The division fur- 
1 This means of expression is necessary in order to avoid the use of the term 
negative. The fact that the insects crawl away from the light end does not prove 
them negative, since when placed in that end they have no other choice. Further- 
more, other results make it unlikely that the flies ever give a truly negative 
reaction. 
